Exchange surfaces
Surface area to volume ratio
As an organism (or a cell) gets bigger, its gets smaller — volume increases faster than surface area as size increases.
Small organisms have a large SA:V ratio, so enough substances can diffuse directly across their body surface to meet their needs. Larger, more complex organisms have a small SA:V ratio, so diffusion across the outer surface alone is too slow — they need specialised and transport systems, like blood, to move substances around efficiently.
Key terms
Features of a good exchange surface
An effective exchange surface has:
- A large surface area, to allow more diffusion to happen at once.
- A thin membrane or wall, giving a short diffusion distance.
- A good blood supply (in animals) or good ventilation (for gases), to maintain a steep concentration gradient.
Together, these features maximise the rate of diffusion across the surface.
Key terms
Alveoli: gas exchange in the lungs
The lungs contain millions of tiny air sacs called , giving the lungs a huge surface area for gas exchange. Each alveolus has a wall just one cell thick, keeping the diffusion distance short, and is surrounded by a network of capillaries that constantly carry blood through, maintaining a steep concentration gradient. A moist lining helps gases dissolve before diffusing.
Oxygen diffuses from the air in the alveoli into the blood; carbon dioxide diffuses the other way, from the blood into the alveoli, to be breathed out.
Key terms
Villi: absorption in the small intestine
The small intestine is lined with millions of finger-like projections called , which greatly increase its surface area for absorbing digested food. Each villus has a wall one cell thick, so the diffusion distance is short, and its own network of capillaries carrying blood away constantly, maintaining a steep concentration gradient for absorption.
Key terms
Gills: gas exchange in fish
Fish take in oxygen from water using their . Water enters through the mouth and passes out over the gills.
Each gill is made of many thin plates called , covered in even smaller folds called . These give a huge surface area for gas exchange. The lamellae have very thin walls, so the diffusion distance is short, and a network of capillaries that constantly carries blood away, keeping a steep concentration gradient.
Oxygen diffuses from the water into the blood, and carbon dioxide diffuses from the blood into the water. Notice it's the same three features again: large surface area, thin walls, and a good blood supply.